1. Field of the Invention
The invention relates to light emitting diodes, and more particularly to light emitting diodes with spreading current and improving light emitting area.
2. Description of the Prior Art
For the photonic semiconductor device, the light emitting diode (LED) may be the most commonly recognized device because of its application to such a wide variety of products and applications such as scientific equipment, medical equipment and, perhaps most commonly, various consumer products in which LEDs form the light source for various signals, indicators, gauges, clocks, and many other familiar items. Semiconductor sources such as LEDs are particularly desirable as light output devices in such items because of their generally long life, low power requirement, and high reliability.
Since early 1970s, gallium nitride (GaN)-based material has attracted attentions in applications to light emitters because of its wide bandgap nature. On the other hand, insulating and lattice mismatched substrate, such as sapphire (aluminum oxide, Al2Oc) with its thermal stability, and optical transparency, has been widely used for the application to Group III nitrides because there is not any substrate having matched lattice. In U.S. Pat. No. 4,153,905, the planner GaN-based LED structure grown on the sapphire substrate with both n-type and p-type layers with corresponding ohmic contacts is proposed. However, it is still difficult to prepare a highly conductive p-type GaN layer. Therefore, it is more difficult to make p-type ohmic contacts on and spread current in the p-type GaN layer.
Recently, both Akasaki et al. and Nakamura et al. have implemented the methods to improve conductivity of p-type GaN layers. However, the conductivity of the p-type GaN is still inferior to that of the n-type one. Consequently, the p-type GaN layer is the topmost layer of the GaN-based LED structure, while a transparent contact on the p-type layer becomes indispensable for the better performance of a device. For the most utility of the light emitting surface, the n-type and p-type contacts have been placed as wide apart as possible, i.e. at the opposite corners of a device.
FIG. 1 is a plan view of a semiconductor light-emitting device having a GaN-based substrate according to the prior art. A cut-off portion 117 is provided in a p-type transparent electrode 115, exposing a portion of the surface of a p-type transparent electrode 115, exposing a portion of the surface of a p-type semiconductor layer 113. A bonding pad 116 strongly adheres to the p-type semiconductor layer 113 through the cut-off portion 117, and is electrically connected with the p-type transparent electrode 115. The cut-off portion 117, and hence the bonding pad 116, are arranged farthest from the n-electrode 114 provided on an n-type semiconductor layer 112. But the arrangement for the contacts has the disadvantage of current crowding occurring between these electrodes. As a result, the light emitting surface is not utilized efficiently, and the lifetimes of the transparent contact and the device are shortened by the current crowding.
One of the objectives of the present invention is to provide light emitting diodes with spreading current and improving light emitting area. A multitude of hollow patterns opened on the transparent contact can block the current shortcuts and further spread current and enhance the light emitting area.
Another one of the objectives of the present invention is to provide light emitting diodes with reducing density of the surface state and leakage current. A passivation layer covering over the surface can reduce the density of the surface state and lower leakage current.
Another one of the objectives of the present invention is to provide light emitting diodes that avoid current crowding. The designed shapes of contacts and position arrangement for contacts can provide the current paths with substantially equal distance between contacts.
The present invention provides a light semiconductor device comprising a substrate and a first semiconductor structure on the substrate. A light emitting structure is on a first portion of the first semiconductor structure. A first contact structure is on a second portion of the first semiconductor structure. The second portion is separated from the first portion of the first semiconductor structure. The first contact structure has a first shape. A second semiconductor structure is on the light emitting structure. A transparent contact is on the second semiconductor structure and has a cut-off portion to expose the portion of the second semiconductor structure and a second shape. A second contact structure is on the cut-off portion of the transparent contact. The second contact structure contacting the second semiconductor has a third shape. The second contact structure with the third shape corresponds to both the transparent contact with the second shape and the first contact structure with the first shape whereby a relationship provides a plurality of current paths with substantially equal distances between the first contact structure and the second contact structure.